FFmpegWebVideo external subtitle support - take 3

this is the minimal patch to get web external subtitles working, plus it fixes the quote-wrapping error.

There's also an alternative version to unify ffmpeg command creation in FFmpegVideo.java which is working for me but is really just a first pass for valib to consider here https://github.com/skeptical/UniversalMediaServer/commit/296b2b1fbec619a4ff567a1ed56664ca63c106f0